Flue gas desulfurization (SO2) and sequestration (CO2) can be achieved by minerals absorption. Mg(OH)2 and Ca(OH)2, as scrubbing agents, are of considerable interests. They possess high chemical reactivity due to their high surface areas. The reaction of Mg(OH)2 with flue gas at its exhaust's temperature (150° C.) and after pre-oxidation of NO to NO2 (the ratio of SO2 to NO2 should be greater than 5), takes place as follows:CO2+Mg(OH)2→MgCO3+H2O  (1)SO2+Mg(OH)2→MgSO3+H2O  (2)SO2+MgSO3+H2O→Mg(HSO3)2  (3)NO2+2MgSO3→2MgSO4+½N2  (4)At 150° C., MgCO3 (Eq. 1) is readily thermally broken down:MgCO3→MgO+CO2↑  (5)MgO (Eq. 5) can be hydrated to Mg(OH)2 and the near pure CO2 can be used, for instance, as a supercritical fluid to enhance heavy oil recovery. The soluble magnesium salts (Eqs. 2-4) can also be regenerated to produce Mg(OH)2 for reuse. As such, the reaction by-products are readily re-generable (MgO) and soluble (magnesium sulfite and sulfate), which are added advantages of using Mg(OH)2 as a scrubbing agent.
Similarly, the reaction of Ca(OH)2 with flue gas takes place as follows:CO2+Ca(OH)2→CaCO3+H2O  (6)SO2+Ca(OH)2→CaSO3+H2O  (7)SO2+CaSO3+H2O→Ca(HSO3)2  (8)NO2+2CaSO3→2CaSO4+½N2  (9)At 150° C., CaCO3 (Eq. 6) is also readily thermally broken down:CaCO3→CaO+CO2↑  (10)and CaO (Eq. 10) can be hydrated to Ca(OH)2. However, the major disadvantage of using Ca(OH)2 as a flue gas scrubbing agent is the generation of the sparingly soluble hydrates of calcium sulfate that requires a further treatment and/or an appropriate disposal path.
The scrubbing of SO2 in a standalone closed loop using calcium chlorofoaluminate or calcium chloroferrate, as a layered double hydroxide rich with Ca(OH)2 crystals, is one of the aspects of this invention.
Alternatively, the use of seawater in a once-through basis to scrub SO2 and covert it to sulfite relies on the appreciable aqueous limit of SO2 and the natural alkalinity of seawater to nearly neutralize the pH of the spent seawater. However, considerable amounts of seawater are needed for SO2 scrubbing. In addition, the spent seawater is richer with sulfite than seawater and thus it's depleted of oxygen. As such, the spent seawater requires: (1) aeration to convert sulfite to sulfate and thus re-adjust the oxygen content; and (2) dilution with further significant amounts of seawater to insure pH neutrality and minimize gypsum precipitation.
The scrubbing of SO2 using the natural alkalinity of saline streams (seawater and the like) as a once-through sink with controlled precipitation of Mg(OH)2 and calcium minerals in a standalone basis; or in conjunction with seawater desalination and/or enhanced oil recovery (injection of de-sulfated saline stream or CO2) are further aspects of this invention.